The present invention relates to the reduction of metal bearing material (e.g., the reduction of iron bearing material such as iron ore).
Many different iron ore reduction processes have been described and/or used in the past. The processes may be traditionally classified into direct reduction processes and smelting reduction processes. Generally, direct reduction processes convert iron ores into a solid state metallic form with, for example, use of shaft furnaces (e.g., natural gas-based shaft furnaces), whereas smelting reduction converts iron ores into molten hot metal without the use of blast furnaces.
Many of the conventional reduction processes for production of direct reduced iron (DRI) are either gas-based processes or coal-based processes. For example, in the gas-based process, direct reduction of iron oxide (e.g., iron ores or iron oxide pellets) employs the use of a reducing gas (e.g., reformed natural gas) to reduce the iron oxide and obtain DRI. Methods of making DRI have employed the use of materials that include carbon (e.g., coal, charcoal, etc.) as a reducing agent. For example, coal-based methods include the SL-RN method described in, for example, the reference entitled “Direct reduction down under: the New Zealand story”, D. A. Bold, et al., Iron Steel International, Vol. 50, 3, pp. 145 and 147-52 (1977), or the FASTMET® method described in, for example, the reference entitled “Development of FASTMET® as a New Direct Reduction Process,” by Miyagawa et al., 1998 ICSTI/IRONMAKING Conference Proceedings, pp. 877-881.
Another reduction process in between gas-based or coal-based direct reduction processing and smelting reduction processing may be referred to as fusion reduction. Fusion reduction processes have been described in, for example, the reference entitled “A new process to produce iron directly from fine ore and coal,” by Kobayashi et al., I&SM, pp. 19-22 (September 2001), and, for example, in the reference entitled “New coal-based process, Hi-QIP, to produce high quality DRI for the EAF,” by Sawa et al., ISIJ International, Vol. 41 (2001), Supplement, pp. S17-S21. Such fusion reduction processes, generally, for example, involve the following generalized processing steps: feed preparation, drying, furnace loading, preheating, reduction, fusion/melting, cooling, product discharge, and product separation.
Various types of hearth furnaces have been described and/or used for direct reduction processing. One type of hearth furnace, referred to as a rotary hearth furnace (RHF), has been used as a furnace for coal-based production. For example, in one embodiment, the rotary hearth furnace has an annular hearth partitioned into a preheating zone, a reduction zone, a fusion zone, and a cooling zone, located along the supply side and the discharge side of the furnace. The annular hearth is supported in the furnace so as to move rotationally. In operation, for example, raw material comprising a mixture, for example, of iron ore and reduction material is charged onto the annular hearth and provided to the preheat zone.
After preheating, through rotation, the iron ore mixture on the hearth is moved to the reduction zone where the iron ore is reduced in the presence of reduction material into reduced and fused iron (e.g., metallic iron nuggets) with use of one or more heat sources (e.g., gas burners). The reduced and fused product, after completion of the reduction process, is cooled in the cooling zone on the rotating hearth for preventing oxidation and facilitating discharge from the furnace.
Various rotary hearth furnaces for use in direct reduction processes have been described. For example, one or more embodiments of such furnaces are described in U.S. Pat. No. 6,126,718 to Sawa et al., issued 3 Oct. 2000 and entitled “Method of Producing a Reduced Metal, and Traveling Hearth Furnace for Producing Same.” Further, for example, other types of hearth furnaces have also been described. For example, a paired straight hearth (PSH) furnace is described in U.S. Pat. No. 6,257,879B1 to Lu et al., issued 10 Jul. 2001, entitled “Paired straight hearth (PSH) furnaces for metal oxide reduction,” as well as a linear hearth furnace (LHF) described in U.S. Provisional Patent Application No. 60/558,197, filed 31 Mar. 2004, published as US 2005-0229748A1, and entitled, “Linear hearth furnace system and methods regarding same.”
Natural gas-based direct reduced iron accounts for over 90% of the world's DRI production. Coal-based processes are generally used to produce the remaining amount of direct reduced iron. However, in many geographical regions, the use of coal may be more desirable because coal prices may be more stable than natural gas prices. Further, many geographical regions are far away from steel mills that use the processed product. Therefore, shipment of iron units in the form of metallized iron nuggets produced by a coal-based fusion reduction process may be more desirable than use of a smelting reduction process.
Generally, metallic iron nuggets are characterized by high grade, essentially 100% metal (e.g., about 96% to about 97% metallic Fe). Such metallic iron nuggets are desirable in many circumstances, for example, at least relative to taconite pellets, which may contain 30% oxygen and 5% gangue. Metallic iron nuggets are low in gangue because silicon dioxide has been removed as slag. As such, with metallic iron nuggets, there is less weight to transport. Further, unlike conventional direct reduced iron, metallic iron nuggets have low oxidation rates because they are solid metal and have little or no porosity. In addition, generally, such metallic iron nuggets are just as easy to handle as iron ore pellets.
One exemplary metallic iron nugget fusion process for producing metallic iron nuggets is referred to as ITmk3. For example, in such a process, dried balls formed using iron ore, coal, and a binder, are fed to furnace (e.g., a rotary hearth furnace). As the temperature increases in the furnace, the iron ore concentrate is reduced and fuses when the temperature reaches between 1450° C. to 1500° C. The resulting products are cooled and then discharged. The cooled products generally include pellet-sized metallic iron nuggets and slag which are broken apart and separated. For example, such metallic iron nuggets produced in such a process are typically about one-quarter to three-eighths inch in size and are reportedly analyzed to include about 96 percent to about 97 percent metallic Fe and about 2.5 percent to about 3.5 percent carbon. For example, one or more embodiments of such a method are described in U.S. Pat. No. 6,036,744 to Negami et al., entitled “Method and apparatus for making metallic iron,” issued 14 Mar. 2000 and U.S. Pat. No. 6,506,231 to Negami et al., entitled “Method and apparatus for making metallic iron,” issued 14 Jan. 2003.
Further, another metallic iron nugget process has also been reportedly used for producing metallic iron. For example, in this process, a pulverized anthracite layer is spread over a hearth and a regular pattern of dimples is made therein. Then, a layer of iron ore and coal mixture is placed and heated to 1500° C. The iron ore is reduced to metallic iron, fused, and collected in the dimples as iron pebbles and slag. Then, the iron pebbles and slag are broken apart and separated. One or more embodiments of such a process are described in U.S. Pat. No. 6,270,552 to Takeda et al., entitled “Rotary hearth furnace for reducing oxides, and method of operating the furnace,” issued 7 Aug. 2001. Further, for example, various embodiments of this process (referred to as the Hi-QIP process) that utilize the formation of cup-like depressions in a solid reducing material to obtain a reduced metal are described in U.S. Pat. No. 6,126,718 to Sawa et al.
Such metallic iron nugget formation processes, therefore, involve mixing of iron-bearing materials and pulverized coal (e.g., a carbonaceous reductant). For example, either with or without forming balls, iron ore/coal mixture is fed to a hearth furnace (e.g., a rotary hearth furnace) and heated to a temperature reportedly 1450° C. to approximately 1500° C. to form fused direct reduced iron (i.e., metallic iron nuggets) and slag. Metallic iron and slag can then be separated, for example, with use of mild mechanical action and magnetic separation techniques.
Other reduction processes for producing reduced iron are described in, for example, U.S. Pat. No. 6,210,462 to Kikuchi et al., entitled “Method and apparatus for making metallic iron,” issued 3 Apr. 2001 and U.S. patent application Ser. No. US2001/0037703 A1 to Fuji et al., entitled “Method for producing reduced iron,” published 8 Nov. 2001. For example, U.S. Pat. No. 6,210,462 to Kikuchi et al. describes a method where preliminary molding of balls is not required to form metallic iron.
However, there are various concerns regarding such iron nugget processes. For example, one major concern of one or more of such processes involves the prevention of slag from reacting with the hearth refractory during such processing. Such a concern may be resolved by placing a layer of pulverized coke or other carbonaceous material on the hearth refractory to prevent the penetration of slag from reacting with the hearth refractory.
Another concern with regard to such metallic iron nugget production processes is that very high temperatures are necessary to complete the process. For example, as reported, such temperatures are in the range of 1450° C. to about 1500° C. This is generally considered fairly high when compared to taconite pelletization carried out at temperatures in the range of about 1288° C. to about 1316° C. Such high temperatures adversely affect furnace refractories, maintenance costs, and energy requirements.
Yet another problem is that sulfur is a major undesirable impurity in steel. However, carbonaceous reductants utilized in metallic iron nugget formation processes generally include sulfur resulting in such an impurity in the nuggets formed.
Further, at least in ITmk3 processes, a prior ball formation process utilizing a binder is employed. For example, iron ore is mixed with pulverized coal and a binder, balled, and then heated. Such a preprocessing (e.g., ball forming) step which utilizes binders adds undesirable cost to a metallic iron nugget production process.
Still further, various steel production processes prefer certain size nuggets. For example, furnace operations that employ conventional scrap charging practices appear to be better fed with large-sized iron nuggets. Other operations that employ direct injection systems for iron materials indicate that a combination of sizes may be important for their operations.
A previously described metallic iron nugget production method that starts with balled feed uses balled iron ore with a maximum size of approximately three-quarter inch diameter dried balls. These balls shrink to iron nuggets of about three-eighths inch in size through losses of oxygen from iron during the reduction process, by the loss of coal by gasification, with loss of weight due to slagging of gangue and ash, and with loss of porosity. Nuggets of such size, in many circumstances, may not provide the advantages associated with larger nuggets that are desirable in certain furnace operations.